U.N. has appealed for $7.7 billion to help alleviate the crisis this year

LONDON—Britain will commit an additional £1.2 billion ($1.75 billion) in humanitarian aid over the next four years to Syria and the immediate region, doubling its commitment, the government said Thursday ahead of a high-level fundraising conference in London.

The Supporting Syria and the Region Conference—co-hosted by Prime Minister David Cameron and attended by U.S. Secretary of State John Kerry—hopes to help answer the United Nations’ appeal for $7.7 billion to alleviate the crisis this year. The conference also aims to...